I'm certainly not going to stop buying from GOG over this one incident. That said, this does reduce the esteem I had held them in. Now I'll have to be on alert about them, where I didn't have to before.
Even though it does make me think less of them, if they limit this kind of nonsense to ephemeral banner ads, I can put up with it. If it expands, though, I'll definitely be less likely to continue spending money with them.